A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).
To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.
Low-k1 lithography may be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula may be expressed as CD=k1×λ/NA, where λ is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size printed, but in this case half-pitch) and k1 is an empirical resolution factor. In general, the smaller k1 the more difficult it becomes to reproduce the pattern on the substrate that resembles the shape and dimensions planned by a circuit designer in order to achieve particular electrical functionality and performance. To overcome these difficulties, sophisticated fine-tuning steps may be applied to the lithographic projection apparatus and/or design layout. These include, for example, but not limited to, optimization of NA, customized illumination schemes, use of phase shifting patterning devices, various optimization of the design layout such as optical proximity correction (OPC, sometimes also referred to as “optical and process correction”) in the design layout, or other methods generally defined as “resolution enhancement techniques” (RET). Alternatively, tight control loops for controlling a stability of the lithographic apparatus may be used to improve reproduction of the pattern at low k1.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a diffraction “spectrum” from which a property of interest of the target can be determined.
At the same time, the known inspection techniques employ radiation in the visible or ultraviolet waveband. This limits the smallest features that can be measured, so that the technique can no longer measure directly the smallest features made in modern lithographic processes. To allow measurement of smaller structures, it has been proposed to use radiation of shorter wavelengths, similar for example to the extreme ultraviolet (EUV) wavelengths used in EUV lithography. Such wavelengths may be in the range 1 to 100 nm, for example, or 1-125 nm. Part or all of this wavelength range may also be referred to as soft x-ray (SXR) wavelengths. Some authors may use SXR to refer to a narrower range of wavelengths, for example in the range 1-10 nm or 1-20 nm. For the purposes of the present disclosure, these terms SXR and EUV will be used without implying any hard distinction. Metrology using harder x-rays, for example in the range 0.1-1 nm is also contemplated. Examples of transmissive and reflective metrology techniques using these wavelengths in transmissive and/or reflective scattering modes are disclosed in published patent application WO2015172963A1. Further examples of metrology techniques and apparatuses using these wavelengths in transmissive and/or reflective scattering modes are disclosed in the published patent applications US2016282282A1, US2017045823A1, WO2017025392A1 and WO2017108404, the latter being international patent application number PCT/EP2016/080058, not published at the present priority date. The contents of all these applications are incorporated herein by reference.
Convenient sources of SXR radiation include higher harmonic generation (HHG) sources, in which infrared pump radiation from a laser is converted to shorter wavelength radiation by interaction with a gaseous medium. HHG sources are available for example from KMLabs, Boulder Colo., USA (http://www.kmlabs.com/). Various modifications of HHG sources are also under consideration for application in inspection apparatus for lithography. Some of these modifications are disclosed for example in European patent application number 16198346.5 dated Nov. 11, 2016, not been published at the priority date of the present application. Other modifications are disclosed in U.S. patent application Ser. No. 15/388,463, published as US2017184511, and international patent application PCT/EP2016/080103, published as WO2017108410. Those two applications both claimed priority from European patent application no. 15202301.6 dated Dec. 23, 2015 and were not published at the priority date of the present application. European patent application no. 16188816.9 dated Sep. 14, 2016, not published at the present priority date, describes the correction of wavefronts in an HHG radiation source to minimize blurring of the measurement spot in an inspection apparatus. The contents of all of these applications are incorporated herein by reference.
As an example of CD measurement using an optical technique, U.S. Pat. No. 5,361,137 discloses a method and apparatus for measuring submicron linewidths, using diffraction gratings. A set of “fixed-linewidth variable-pitchwidth” test gratings has a number of gratings, each grating having the same linewidth but having different pitchwidths. These gratings are illuminated to form diffraction patterns. A set of peak intensities of the first or second order diffraction image from each grating is recorded. Either of these intensity values forms a curve around an extrema, which represents the intensity from a grating whose pitchwidth is equal to one-half the linewidth.
Lithographic patterning performance is now being driven by edge placement errors (EPE). The position of the edge of a feature is determined by the feature's lateral position (Overlay) and the size of the feature (CD). Part of this is very local and stochastic in nature. E.g. Line Edge Roughness (LER) results in very local CD variations. It is expected that for future lithography nodes this local CD uniformity (LCDU) will become a large, dominant contributor to the EPE performance. Since LCDU will become so important, it will become important to control it and minimize it, by optimizing the process conditions. As a first step in this it is necessary to measure and monitor the LCDU.
Although linewidths are measured using various known optical techniques, LCDU and LER parameters are currently obtained by means of CD-SEM (CD Scanning Electron Microscope) inspection technologies. Therefore, compared to optical techniques, only a sparse sampling can be used, or the measurement time explodes if a large number of LCDU parameters must be obtained.